This invention relates to a device for driving an actuator for tracking-controlling, focus-controlling or otherwise controlling an optical pickup in an optical type recording and reproducing device such as a CD (Compact Disc) player, a CD-V (Compact Disc with video) player and an LV (laser vision) player and, more particularly, to a device of this type which has reduced unnecessary vibration occurring due to transient response of an optical pickup and thereby facilitates control of the actuator.
For reading a signal recorded on a disc, an optical pickup of an optical type disc playback device generally comprises actuators performing controls including tracking control, focus control and tangential control. These actuators are generally constructed of coils and magnetic circuits and each is driven by a servo system based on a detection of an error signal relative to a normal state in response to a detected signal from the disc so as to control a beam spot to be correctly positioned on the disc.
As an example of such an optical pickup devices is one having a symmetrical drive type tracking actuator as shown in a plan in FIG. 2. This optical pickup 10 comprises a lens support 12 which is horizontally rotatably supported on a rotation shaft 14. An objective lens 16 is fixed at an end portion of the lens support 12. This objective lens 16 swings clockwise and counterclockwise in accordance with the pivoting of the lens support 12 in the horizontal direction and tracking thereby is performed. Cores 18 and 20 are formed on both sides of the lens support 12 and tracking coils 22 and 24 are wound on these cores 18 and 20. The cores 18 and 20 are separated from each other by the lens support 12 so that mutual inductance is not generated between the tracking coils 22 and 24. To a stationary portion of the optical pickup 10 is secured magnets 26 and 28 opposite to the cores 18 and 20. To both sides of the lens support 12 is secured a damper 30 which is made of rubber or other material and formed substantially in the shape of "8". Annular portions 30a and 30b are formed in both end portions of the damper 30 and damper supports 32 and 34 fixedly provided on a stationary portion of the optical pickup 10 are fitted in the annular portions 30a and 30b to support the damper 30.
By applying a tracking error signal to the tracking coils 22 and 24, the lens support 12 is pivoted by an angle corresponding to the error signal about the rotation shaft 14 thereby causing the objective lens to swing clockwise or counterclockwise to perform tracking.
FIG. 3 shows a prior art tracking actuator driving device employing the optical pickup of FIG. 2. A disc 44 is driven and rotated by a spindle motor 46. In the optical pickup 10, laser beam 38 produced by a laser diode 36 is radiated on the recording surface of the disc 44 through a grating 40, a beam splitter 42 and an objective lens 16 and the reflected beam is received by a four split photo-detector 50 through the objective lens 16, the beam splitter 42 and a cylindrical lens 48.
Outputs of the four split photo-detector 50 on diagonals are added together. These diagonal sum signals are further added by an adder 52 whereby an RF signal is produced and reproduction of a signal is made.
These diagonal sum signals are also supplied to a differential amplifier 54 and a tracking error signal is produced. This tracking error signal is supplied to tracking coils 24 and 22 connected in series to each other through a compensation amplifier 56, a tracking loop switch 58 and a drive amplifier 60 to pivot the lens support 12 and thereby cause the objective lens 16 to swing clockwise or counterclockwise to perform tracking control.
Since the actuator of the optical pickup comprises an objective lens and other elements having a relatively large mass, it has to adopt a design which inevitably produces vibrations due to transient response of the optical pickup. For reducing vibrations due to a transient response, the damper 30 as shown in FIG. 2 is generally used. This damper 30, however, cannot sufficiently reduce vibrations but, rather, makes resonance characteristic conspicuous in frequency response (open loop) as shown in FIG. 4. This resonance characteristic is a factor which is hard to control and makes the tracking servo unstable and is called unnecessary vibration characteristic. For removing this unnecessary vibration characteristic, the prior art device employs a servo system using a detection signal from the disc and also employs the phase compensation amplifier 56 in its servo loop for phase-compensating.
For performing this phase compensation, complex parameters forming this resonance characteristic must be analyzed by using a large sized computer and a complicated phase compensation must be applied in accordance with result of the analysis by the computer. It is however extremely difficult to analyze these parameters accurately and realize an accurate phase compensation in accordance with these parameters.
Even if the parameters are analyzed accurately and an accurate phase compensation is applied in accordance with these parameters, characteristics of the conductor of a coil, compensation circuit and damper undergo change due to external conditions such as ambient temperature which changes with a wide range between -25.degree. C. and +75.degree. C. as well as aging with resulting change in the frequency response as shown in FIG. 4, so that it is extremely difficult to maintain an optimum phase compensation constantly. Besides, phase compensation designed to be optimum under some characteristics conditions sometimes becomes a factor which adversely affects the phase characteristics under other characteristics conditions.
The servo loop shown in FIG. 3 works on the condition that there exists reflected laser beam. If, therefore, a reflected beam is not obtained due to scratch, dust or the like on the recorded surface of the disc, the servo loop is virtually cut off and there will occur a case where the lens support 12 supported by the damper 30 is put in a free vibration state and also in a resonant state with a result that the lens support 12 vibrates at an abnormal amplitude (this may also be called a kind of unnecessary vibration). Such vibration tends to give rise to inconvenience that, when the servo loop resumes its operation after reflected beam is obtained again, a wrong track, i.e., a track which is different from one which was being traced before interruption of the reflected beam, is caught by the optical pickup. This makes the tracking control even more difficult. The above described phenomenon occurs also when the servo loop is cut off by turning off the tracking loop switch 58 such as when compulsory moving of the optical pickup such as search is made or when track jump is performed by generating a jump pulse from a jump pulse generator 62 in FIG. 3.
It is, therefore, an object of the invention to provide an actuator driving circuit for an optical pickup which has eliminated the above described problems of the prior art device and is capable of reducing unnecessary vibration due to transient response of the optical pickup and thereby facilitating control of the actuator with a relatively simple structure.